Noise squelch system



May 28 1946- R. A. PETERSON 2,400,948

NOISE SQUELGH SYSTEM Filed March 19, 1945 ATTORNEY trol. ers, to utilizethe grid current flowing in the Patented May 28, 1.946

UNITED ls'ra'rlezsl PATENT ortica .i

NOISE SQUELCH SYSTEM Robert A. Peterson, Haddoniield, N. J., assignertol Radio Corporation of America., a corporation of Delaware ApplicationMarch 19, 1942i, Serial No. 479,727 4 claims. (Cl. 25o- 20) My presentvinvention relates to noise squelch systems for radio receivers, and moreparticularly to a noise-actuated control system for a receiver of anglemodulated carrier wave energy.

In the past various schemes have Ibeen proposed forimpairing thereproduction of a radio receiver in response to a low signal-to-noiseratio at the receiver signal collector. In the case o f an anglemodulated carrier wave receiver, such as a receiver of frequencymodulatedcarrier energy, it has been proposed to use thediscriminatorrectifier network output voltage for noise con.

It has, also, been proposed in such receivlimiter grid circuit. However,these various prior circuits have not been suiiiciently sensitive,'reliable or eiicient to warrant widespread use thereof.

I It is one of the main objects of my present` l lto squelch, or mute, amodulation signal amplier stage. Y

Another object of my invention is to provide an improved muting circuitfor an FM receiver, or a phase modulated (PM hereinafter for brevity)carrier wave receiver, which mutes the audio outputof the receiver whenno signal is present at the receiver input terminals, and which permitsthe receiver to reproduce the modula` tion in eiilcient manner when asignal of proper strength is received.

Still another object of my invention is to provide a noise squelchcircuit for an FM or PM receiver; the squelch circuit comprising adirect current voltage amplier which acts to .apply a cut-off bias to anaudio ampler when the former is conductive, the noise voltage developedin the plate circuit of the limiter tubev being recti-J fled' toprovidey a.V bias control voltage for the direct current voltageamplifler, and the-rectied noise voltage being opposed by a negativevoltage derived from :the limiter grid. 'circuit thereby to provide aresidual Apositive voltage which is applied as said bias controlvoltagein the absence 4of a usable signal.

Still other objects of `this invention are to improve the sensitivityandeiliciency of noise squelch circuits vof receivers of the anglemodulated type, and more especially to provide a receiver whose noiseoutput is controlled by an economical circuit which can be readilymanufactured and assembled.

The novel features which I believe to be characteristic of my inventionare set forth with particularity in the appended claims; the inventionitself, however,V as to both its organization and method of operationwill best be understood by reference to the following description, takenin connection with the drawing', in which I have indicateddiagrammatically a system whereby my invention may -be carried intoeiect.

Referring to the accompanying drawing, wherein there is shown a portionof a superheterodyne receiver of the VFM type essentialV to a properunderstanding of my invention, it is pointed out that the usual networksprior to-the schematiy cally represented intermediate frequency (I. F.)amplifier I :are omitted. These networks are well known to those skilledin the art. Usually they comprise a signal collector device, such as agrounded antenna or a dipole, one or more tun'- able radio, frequencyamplifiers, and a converter followed by one or more I. F. amplifiers.The receiver frequency may be any frequency that .is desirable for anangle modulated `carrier Wave receiver. vThe receiver frequency band maybe in the 26-50 megacycle (mc.) range', or in the 'I0-100 mc. range.,The operating I'. F. value may :'be in the mc. range, or it may be inthe kilocycle (kc.) range. Wherethe latter is the case, doubleconversion can be used to reduce the car- 'rier frequency down to an I.F. value of, for example, 455 kc. Let it be assumed, then, that prior toamplifier I there has `been employed double conversion of the modulatedcarrier energy, and that the signal fed to stage I has an I. F. value'of455 kc. The invention, however, is not limited to any specific frequencyvalues. l

The signal energy may be FM or PM. The generic term angle modulated hasbeen heretofore employed, and will be used in the'fol- .lowingdescriptionand claims to indicate either PM or hybrids of the two. Byway of specific example, itisv assumed that the received signal ,energyis FM waveenergy. It is well known to those skilled'lnthe artl ofreceiving FM waves that the noise output of the receiver rapidlyincreases when the signal-to-noise-ratio at the signal collector devicefalls below a predetermined value. ratio will decrease below thenoise-free Value when tuning between stations, when moving an excessivedistance from a desired FM transmitter, or when the receiver is movedinto a. shielding area such as a bridge and the like.'` During suchperiods of noisy reception it is desirable to mute the receiver output.In the vously used in FM receivers have employed the reduction in-noisevoltage at the demodulator output circuit in response to signalreception to operate the squelch circuit. However, such an arrangementhas not proved to besufiiciently sensitive. It is, also, desirable toprovide a muting circuit which will give improved operation in thepresence of ignition noise interference.

The present noise muting arrangement satises all reasonable requirementsin an FM receiver. Generally speaking, my invention is based. on thefact that the usual limiter tube circuit provides in its plate circuitnoise voltage components in the absence of signal energy of a usablevalue. I utilize these noise voltage components to .provide cut-o biasfor a signal transmission stage of the receiving system.

Referring to the specific circuits of the drawing, the numeral 2designates the limiter tube. While the tube is shown as of the pentodetype. it will be understood that any other suitable type of tube may beemployed. The limiter tube has its input grid 2' connected to groundthrough a path which comprises the secondary winding 3 of the I. F.transformer 4 and the resistorRz. Each of the primary and secondarycircuits of I.- F. transformer 4 is tuned to the operating I. F. value.The condenser C1 shunts the resistor R2. The network Ra-Ci functions asthe usual signalresponsive grid biasing network to provide grid circuitlimiting action for input signals whose amplitude exceeds apredetermined amplitude. The network Rz-Ci has a relatively small timeconstant value. The cathode of tube 2 is grounded, while the screen gridthereof is connected to a point of positive direct current potentialthrough resistor R1. The latter is grounded for I. F. currents bycondenser Ca.

The plate of the limiter tube is connected to the +B terminal of thedirect current source by means of .the primary winding 6 of the'transformerfeeding the discriminator. The limiter operation is wellknown to those skilled in the art. Its ideal input vs. outpucharacteristic is depicted above tube 2. The function of the limiter issubstantially to eliminate amplitude variations from the FM signalenergy applied to the discriminator circuit. l

The FM detector network itself may be of any well known form. -Thespeciilc detector illustrated is of a well known and simple type.Essentially it comprises a pair of'diodes which are y connected inopposed relation,. and whose tuned inpuI-,Ucircuits are equally andoppositely mistuned in frequency with respect to the mean frequency ofthe applied signal-energy. The detector tube 'I is shown as "of the 6H6type by way of example.

One diode section 8 has connected between its electrodes the seriesarrangement of the input circuit S and the output load resistor l0.'I'he second diode section 8' has connected between its electrodes -theinput circuit 9' and the output load resistor I0'. The cathode of diodesection 8' is grounded, as isl the case with the lower end of loadresistor I0. The resistors I0'and III' are in series, and each of themis bypassed by appropriate condensers for I. F. currents.

The input circuits 9 and 9' are each reactively coupled to the primarycircuit 6, but there is very` little coupling between-the -two inputcircuits 9 and 9. The reference characters F1 and F2 denote therespective off-tune frequencies of these two circuits. The meanfrequency of the applied signal energyis indicatedA by the symbol Fc,and in the present case the value of Fc is the operating I. F. value.Above the FM detector there is depicted the ideal frequency deviationVs. rectified output characteristic of the detector. This-is the wellknown inclined characteristic curve of the FM detector. The .peaks ofthe detector characteristic may be spaced, if desired, by a distancegreater than the maximum frequency deviation of the FM signal energy.Itl

is not believed necessary to describe the functioning of the detector.circuit, sinceit is very well known to those skilled in the'art. It issufficient to indicate that there will be taken off from the upper endof resistor I0 the modulation signal, in this case audio frequency,which was originally applied to the carrier at the FM transmitter.

Where the signal energy to be received is a PM carrier wave, itvmaystill be necessary to utilize an amplitude limiter prior to detection.The PM detection network will be of any well known type. It issufficient for the purposes of this application to point out that theessential difference between a PM carrier wave and an FM carrier waveresides in the fact that in the former case the extent of frequencydeviation is greater for the higher audio frequencies.

The modulation signal utilization network is indicated as comprising atube Il, as for example a triode. whose cathode I2 is connected toground through a biasing resistor I3 in series receives its bias fromthe voltage .across resistor The biasing of tube II is controlled by thedirect current voltage amplier I1 which is shown, by way of specificillustration, as a triode. The plate I8 of tube I1 is connected to thegrid of tube I'I through a resistor I9, the lower end of resistor I9being bypassed to ground for pulsating current components bycondenser20. The plate I8 is connected through resistor 2| to the upper end ofresistor I3", and the cathode of tube I1 is grounded. A positive voltageexists across I3", and this is used as plate voltage for tube I1. Thecontrol grid 22 of tube I 1 is returned back to ground through a pathwhich the opposite ends thereof bypassed to ground denser 23'. 'I'heresistor 24 acts as the output load of a voltage doubling rectifierwhich comprises the GHS type tube 30.`

The'diode section 3| of tube 30 has its anode connected to the upper endof resistorv R1 by the coupling condenser; 32, while the cathode of thediode section 3l' is connected to the junction of resistors 24 and 23.The diodeI section 33 of tube 30 has its anode connected to the left endof'resistor 24, while its cathode is connected to the output terminal ofcoupling condenser 32. The noise voltage vcomponents developed acrossthe resistor R1 are transmitted through the coupling condenser 32 to thediode sections 3| and 33 of tube 30. By virtue of the voltage doublingconnections of the diode sections 3| and 33 with respect to outputresistor 24, there will be developed across the resistor 24 rectifiednoise voltage whose amplitude is substantially double the amplitude ofthe noise voltage components at R1.

The rectied noise voltage is applied to the grid 22 of tube in apositive polarity sense. The network 23-23' functions as a filter tosuppress any pulsating voltage components which a point on R2A such thatnegative voltage developed across Rz will be su-fliciently less thanthepositive voltage developed across resistor 24|y to provide a residualpositive bias for grid 22, This residual positive bias will exist, ofcourse,

through 4condenser 32. However, when the may .exist in the outputvoltage across resistor y duces the positive voltage developed acrossresistor 24 to the proper value. l

When the signal-to-noise ratio at the receiver Vcollector device risesto a usable value, the noise voltage components at R1 rapidly decrease.This means that the voltage developed across resistor 24 rapidlydecreases. However, the voltage across R2 does not decrease. It eitherwill remain constant, or it will actually increase. This means that thegrid 2'2 of the tube |1 will be biased sufficiently negative to cut offits space current iiow,

and the cut-off bias across resistor 2| will be removed from the grid ofaudio frequency amplier If an additional limiter tube-is employed priorto thelimiter tube 2, the voltage drop across Ra will be invariable ifthe no-signal noise originating in the iirst circuits of the receiverproduce enough voltage to saturate the first limiter,

"tube'H completely muted in the absence of received signal energy.'However, for received signals of 0.1 microvolt, or greater, itis,desirable to have the cut-off bias removed from the grid of tube l.Of course, these constants are purely illustrative, and the set designercan adjust the vconstants to suit the particular requirements oftransmission.

` 'only if suiilcient noise voltage is transmitted residualpositivevoltage applied to grid 22 has a y sufficient and predeterminedmagnitude tube I'l will be conductive, and will provide spacecursuiicient voltage to cut oi the space current now in tube |'l byvirtue oi' the fact that the voltage across resistor 2| is applied in anegative polarity sense to the grid of tube Il. The condenser 20 acts todelay the change in bias across resistor 2|.

In the no-signal state it is desirable to adjustthe slider 26 onresistor R2 so that the voltage ap- While Ihave indicated and describeda system for carrying my invention into eiiect, it will be apparent Vtoone skilled in the art that my invention is by no means limited to theparticular ortype including an amplitude limiter stage having plied togrid 22 will be slightly positive thereby causing the audio amplifiertube Il to be cut oif. This means that there will be no noisereproduction at the receiver reproducen. It is pointed out that thenoise voltage components produced in the plate circuit .of the limitertube may go up to several kilocycles in frequency, depending on thebandwidth' of the I. F. amplifier. In other words, the limiter tube actsin the `the limiter output electrodes, a load resistor connected betweenthe cathode of said one diode and anode of the second diode forproviding a. rectiiied voltage whose magnitude is proportional to ynoise voltage appearing in the limiter output cirmanner oi' a detectorfor the radio, frequency 1 noise components so far as its plate circuitis cuit in response toa substantial reduction in the signal-to-noiseratio at the receiverl input terminals, a resistor-condenser network incircuit potential of said network to saidload resistor concerned, and,hence, develops across its plate vThe resistor 25 acts to isolatenetwork Ra-Ci from condenser 21. It is this recti'cation which providesthe'opposing negative voltage that refor combining the voltage output ofthe rectiiier and said control voltage to provide a residual voltagewhich is positive, an electron discharge device responsive to saidpositive voltage for producing a space current, and means responsive -tothe last-named space current for rendering said 4input and outputelectrodes, a ireeuency modulation detector and a modulation signalamplifier;

tlhe improvement comprising a rectifier-doubler coupled to the limiteroutput electrodes for providing a rectined voltage whose magnitude isprof portional to noise voltage appearing in the lim- 4iter outputcircuit, in response to a substantial reduction in the signal-to-noiseratio at the receiver input tenninals, a resistor-condenser network forderiving from the input limiter elec-V trodes a control voltage Whosemagnitude depends upon the amplitude of received signal energy, meanscombining the voltage output of the rectier-doubler and said controlvoltage to provide a residual voltage which is positive, an electrondischarge device responsive to said positive voltage for producing aspace current, and means responsive to the last-named space current forrendering said modulation signal amplier inopera` tive.

3. In a frequency modulation receiver of the type including an amplitudelimiter tube having input and output electrodes, a frequency modulationdetector and a modulation signal amplier tube; the improvementlcomprising a rectier con- ,sisting of opposeddiodes having a commoncou-V pling to the limiter output electrodes, an output means responsiveto said positive voltage for producing a space current flow in saiddevice, and resistive means, connected'between the amplier inputelectrodes and responsive to the last-named space current, for renderingsaid modulation signal amplifier tube inoperative.

4. In a frequency modulation receiver of the type including asignaltransmission tube having input and output electrodes, a frequencymodulation detector coupled to the output 'electrodes and a modulationsignal amplifier coupled to the detector; the improvement comprising apair of opposed diodes, a load resistor common to the space currentpaths of the opposed diodes, a condenser connecting unlike electrodes ofsaid diodes to said output electrodes for providing a rectiied voltageacross said load resistor whose magnitude is proportional to noisevoltage, a resistor-condenser network connected to said input electrodesfor producinga control Vvoltage whose magnitude depends upon theamplitude of received signal energy, means comprising a connection fromthe negative potential end of said load resistor to a slider adapted tobe adjusted along the resistor of said resistor-condenser network forcombining the voltage output of the rectier and said control voltage toprovide a residual voltage which is positive, an amplifier tube havingan input connection responsive to said positive voltage for producingspace current ow, means responsive to the last-named space current forrendering said modulation sigmeans combining in polarity opposition thevoltage output of the rectifier and said control voltage to provide aresidual direct current voltage which is positive, an electron dischargedevice,

nal amplifier inoperative, said .last means comprising a resistorconnected between the modu- `als lation ampliiier input electrodes andadapted to be traversed .by said space current.

ROBERT A. PETERSON.

